It is known to manufacture hollow metallic aerofoils, for example to be used as blades in a jet engine, by superplastic forming and diffusion bonding metallic panels, the panels forming pressure and suction surfaces of the blade. Such blades are widely used in the civil aerospace industry and may also be used in blisks, particularly in military applications. The metallic panels may include elementary metal, metal alloys and metal matrix composites. At least one of the metallic panels must be capable of superplastic extension. In one known process the surfaces of the panels to be joined are cleaned, and at least one surface of the one or more of the panels is coated in preselected areas with a stop-off material to prevent diffusion bonding. The panels are arranged in a stack and the edges of the panels are welded together, except where a pipe is welded to the panels, to form an assembly. The pipe enables a vacuum, or inert gas pressure, to be applied to the interior of the assembly. The assembly is placed in an autoclave and heated so as to “bake out” the binder from the material to prevent diffusion bonding. The assembly is then evacuated, using the pipe, and the pipe is sealed. The sealed assembly is placed in a pressure vessel and is heated and pressed to diffusion bond the panels together to form an integral structure. Diffusion bonding occurs when two mat surfaces are pressed together under temperature, time and pressure conditions that allow atom interchange across the interface. The first pipe is removed and a second pipe is fitted to the diffusion bonded assembly at the position where the first pipe was located. The integral structure is located between appropriately shaped dies and is placed within an autoclave. The integral structure and dies are heated and pressurised fluid is supplied through the second pipe into the interior of the integral structure to cause at least one of the panels to be superplastically formed to produce an article matching the shape of the dies.
In addition to the hollow assembly just described, it is also known to insert a membrane between the metallic panels prior to the above described process. The location of diffusion bonds between the membrane and the adjacent panels can be controlled by applying the stop-off material to preselected areas on each side of the membrane (or respective panels). When the aerofoil is subsequently expanded, the membrane adheres to the panels where the diffusion bond is allowed to form and thereby provides an internal structure. The internal structure is provided to increase the strength and stiffness of the aerofoil and also to prevent lateral flexing of the panels, referred to as “panting”.
The assembly may be filled or part filled by a suitable material to provide damping of the structure and therefore to reduce vibration. A suitable material may be one which possesses viscoelastic properties. Viscoelasticity is a property of a solid or liquid which when deformed exhibits both viscous and elastic behaviour through the simultaneous dissipation and storage of mechanical energy. A known method is to introduce a viscoelastic material, for example a Huntsman™ syntactic damping paste or some such similar product, into the cavity by injecting or otherwise introducing the material into some or all of the cavity. This technique may be applied in a hollow assembly wherein the cavity is smooth walled with no internal structure, as disclosed for example in GB2371095. In this configuration the viscoelastic material is restrained solely by the bond between the viscoelastic material and the walls of the cavity. If this bond is not sufficient to retain the viscoelastic material during working conditions, in particular centrifugal loading, then, since the viscoelastic material is a parasitic mass which is unable to support its own weight, the hydrostatic load of the unrestrained material can cause the blade to fail rapidly. Accordingly, the consequences of failure of this bond are severe. It is therefore desirable to provide some form of mechanical keying as an alternative or additional means of retaining and restraining the viscoelastic material. An internal structure, for example as described above, may be used to provide such a restraining or retaining effect on the injected material. However by providing a rigid internal structure the benefits of damping the aerofoil may be reduced as the aerofoil is less flexible as a result of the internal structure. This may lead to additional problems where the aerofoil prematurely fatigues or cracks as a result of the reduced flexibility. Other configurations use internal ribs, which may be attached to alternate interior walls of the aerofoil but which are not connected to one another, as disclosed for example in GB2450934. This configuration permits damping of the assembly whilst the re-entrant features still provide a means of retaining the injected material. Other methods use dual membranes to produce a lightweight internal structure in the aerofoil. Internal membranes or ribs may be mounted on protrusions created on the panels of the assembly during forging and transferred to the internal cavity during superplastic forming as described for example in GB2450934.
Known internal structures are such that they may advantageously bear a significant load under normal working conditions, thus allowing the thickness of the panels to be reduced and the size of the cavity to be increased. Also the internal structure may provide additional resistance to a bird strike or impact from ice or other foreign objects. However the use of an internal structure to physically restrain the viscoelastic material adds weight to the aerofoil and thus increases the stresses on the aerofoil, in particular at the root of the aerofoil. This increases the blade off energy if the blade were to fail, which must be taken into account when designing the blade retention system. In addition the provision of complex internal structures increases manufacturing costs and lead times. It is therefore desirable to provide an improved method of restraining a viscoelastic material within a cavity which addresses some or all of the above problems associated with the prior art methods.